1. Field of the Invention
The invention relates to head restraints forming the upper portion of a vehicle seat backrest that may be deployed, for example, to a relatively-forward position in the event of a rear collision in order to increase vehicle occupant safety.
2. Background Art
The prior art teaches an automotive seat that includes a seat cushion, and a backrest pivotally mounted to the seat cushion so that the backrest may be reclined at a selected angle relative to the seat cushion for comfort. To improve seat comfort, a headrest is often defined on the upper portion of the backrest, for example, by supporting a cushion atop the backrest using a pair of posts that are slidably received in complementary guides defined in the upper portion of the backrest frame.
To reduce the likelihood of an undesirable over-extension of the occupant's cervical vertebra in a rear collision, the prior art teaches pivoting the headrest about a pivot axis positioned somewhat vertically beneath the headrest to a relatively forward position within the vehicle in response to such a rear collision. In this manner, the distance which an occupant's head must otherwise travel before being accelerated through contact with the headrest is reduced. For example, in U.S. Pat. No. 5,378,043, the headrest is pivoted relative to the seat backrest using a lever mechanism coupled to a pressure plate disposed within the backrest beneath its upholstery. When the pressure plate is actuated by occupant inertial forces during a rear collision, the upper portion of the backrest or the headrest-supporting guides pivot about a predetermined axis to bring the headrest to a forward position within the vehicle.
Co-pending U.S. patent application Ser. No. 09/938,416 filed Aug. 23, 2001, which is assigned to the assignee of the invention, alternatively teaches a “clamshell” headrest assembly that includes an angularly-fixed, rearward base portion rigidly mounted atop the backrest on a pair of downwardly-extending posts that are slidably received in the backrest frame. The clamshell headrest assembly further includes a forward, occupant-engaging or “contact” portion that is pivotally mounted to the base portion along the base portion's upper edge. A compression spring operates to bias the lower edge of the contact portion toward the seated occupant, while a spread-lever arrangement operated by a tensioned Bowden cable draws the lower edge of the contact portion back against the base portion. When the occupant exerts a threshold occupant inertial force against a pressure plate disposed within the backrest, the Bowden cable is released to permit the “clamshell” to open, as urged by the compression spring. The lower edge of the headrest's contact portion is thus urged toward the occupant's head.
Significantly, because the contact portion of the headrest advances toward the occupant's head through rotation about its upper edge, the inclination of the headrest's engaging surface similarly changes during such an advance, with the degree of advancement of any particular point on the headrest's engaging surface being inversely related to its relative height above the seat cushion. The uppermost part of the headrest's contact portion (proximate to the pivot axis) remains essentially stationary as the contact portion advances upon release of the tensioned Bowden cable.
In yet another prior art approach, U.S. Pat. No. 6,082,817 teaches a multi-link system for supporting the contact portion of the headrest relative to a fixed base portion. Upon the tensioning of a Bowden cable, for example, using a pyrotechnic charge, the contact portion moves both forwardly and upwardly as the links rotate about respective, parallel-spaced axis. While the use of multiple links can effect a forward movement of the forward part of the headrest without an attendant change in the engaging surface's inclination, the relative height of the engaging surface changes during the deployment, thereby presenting the possibility that the contact portion of the headrest will not achieve a desired engagement with the occupant's head.